The present invention relates to a process for preparing a magnetic disk by forming a magnetic continuous thin film on the surface of a substrate. More particularly, the present invention relates to a process for preparing a magnetic disk improved by dissolving problems concerning the friction coefficient of the surface and the sticking to a magnetic head.
A magnetic recording medium using a magnetic continuous thin film (hereinafter referred to as a "magnetic thin film") as a high density recording material is not widely used because of the problems in respect of mechanical durability and the like.
Namely, a recording medium using a magnetic thin film and having a very smooth surface is likely to lead to sticking when brought in contact with a magnetic head. Further, a liquid lubricant applied to the medium is easily removed by the contact with the magnetic head, whereby the friction coefficient increases, thus leading to head crush.
In order to avoid such problems, a mechanical texture method has been attempted in which scratch marks are mechanically imparted to the surface of the substrate by means of e.g. sand paper. However, it is very difficult to impart such scratch marks while controlling not to increase bit errors and not to cause the sticking to the magnetic head. Further, in the mechanically scratched texture, fine burrs exist on the surface, and they tend to peel off upon collision with the magnetic head and thus lead to head crush.
Japanese Unexamined Patent Publication No. 22220/1984 discloses the invention relating to a process for preparing a substrate for a magnetic disk, the improvements of which are to reduce the friction coefficient of the surface of the magnetic disk having a magnetic thin film formed on the substrate and to reduce a sticking force to a magnetic head.
This process comprises subjecting the surface of an aluminum alloy substrate coated with an Alumite layer to mirror surface-finishing, and etching the mirror surface-finished surface to form concave parts having a surface precision of Ra 70-1,400 .ANG..
The mirror surface-finishing in the cited process can be conducted by abrading, rubbing, polishing or other methods, thereby forming the mirror surface-finished surface having R.sub.max of at most 200 .ANG. and Ra of at most 50 .ANG., but in this state, the sticking of a magnetic head occurs. In order to avoid the sticking of the magnetic head, the surface is etched to form convex parts in such manner as to have a surface precision of Ra 70-1,400 .ANG., but this process still has the following problems.
Firstly, the pore area ratio (ratio of the total area of pores to the entire surface area) is constantly about 10% before forming a magnetic thin film since the pores of the Alumite layer are not widened by the mirror surface-finishing and etching steps. Consequently, the contacting area of a magnetic head is relatively large, and it is therefore necessary to enlarge the surface roughness in such manner as to reduce the friction coefficient.
Secondly, the etching is plasma-ecthing carried out in the atmosphere of O.sub.2, Ar.sub.2 or a mixture thereof, and the plasma-etching abrades particles out of the Alumite layer surface by bumping plasma against the Alumite layer. Accordingly, after etching, the surface becomes rough and the surface roughness remarkably varies due to the presence of the abraded powder (SiO.sub.2, Al.sub.2 O.sub.3) in Alumite pores and the impurities (Fe.sub.3 Si and the like) in the Alumite film, which respectively have different etching rates.
In other words, according to the above process, the plasma-etching is employed to secure the required surface precision taking the small pore area ratio into consideration, but the surface roughness after etching remarkably varies, which consequently leads to large variation in the friction coefficient over the entire surface of the magnetic disk. Thus, it is difficult to prevent the sticking of a magnetic head and the head crush, and it is also difficult to reduce the spacing between the magnetic head and the magnetic disk.